I need help converting a battery system

Thanks for helping in advance!

:lol:
Here is my project:

I have a Power wheels Car for my daughter that I would like to convert to a 12V DC system! So I may run off a motorcycle battery!

Here is what I know:

The battery that are currently being used are rechargable lead fill (2) 6V ,9.5 ampere hour, 4 amp.
The batteries run in series to 12v (+ of one batt. to the - of the other)
The car has a shifter that goes 3 speed. 2 forward (low and high) plus 1 reverse speed. There is a foot pedal to engage the motors.

Please offer ideas as to how I can create this conversion.
Thanks, D

If the two 6V batteries are in series, and there's no other connection to their centre tap, it's already a 12V system - simple replace the two batteries with one.